1. Field of the Invention
The present invention relates to a fluid measuring device comprising flow rectifiers to determine quantities of fluid flow or flow rates precisely.
The term "flow rectifier" used herein means a device for restraining an occurrence of turbulence in a fluid flowing therethrough as little as possible to obtain a steady flow of the fluid.
2. Description of the Prior Art
In engines for automobiles or the like, electronically controlled fuel supply systems have been investigated and developed for controlling amounts of fuel supply by electrically detecting intake air flows in the engines in order to more lower the fuel consumption and more improve the exhaust gas composition.
As means for detecting quantities of air flows, there has been Karman vortices flow meter which directly or indirectly counts vortex rows to obtain a quantity of the air flow, which vortex rows alternately occur proportionally to a flow rate of the fluid to be measured and at a location downstream of an obstruction (for example a column-like body) located in a passage for the fluid.
In such a flow meter, however, errors often arise in measurement when the flow is disturbed to reduce the vortices. It is particularly acute in a great quantity of flow to lower the reliability of the flow meter. In order to effect a correct measurement of flow, therefore, it is required to introduce the flow into a flow meter under a sufficiently stable condition.
With an application of this flow meter to an air intake system for an automobile engine, an intake air hardly flows under a steady condition because of pulsations in the intake air and unavoidable curved or bent portions of the intake passage due to relative positions of other engine component members, so that the measured values generally include considerably great errors.
In order to suppress the disturbance in flow, it has been known to arrange a flow rectifier upstream of the flow meter. However, as the flow rectifier generally causes losses of pressure, an air intake system for an engine including a flow rectifier arranged therein would lower an output of the engine, so that the flow rectifying effect can be achieved only at the sacrifice of the engine output. In the prior art, moreover, as a single flow rectifier is located upstream of a flow meter, an accuracy of measurement is not improved so much by the flow rectifying effect notwithstanding increased losses of pressure due to the rectifier, with resulting increased air intake resistance.
In case of a wire screen which has been used as a flow rectifier, particularly, losses of pressure are remarkably increased because very fine mesh screens having opening ratios of in the order of 20-30% are generally used for this purpose in order to obtain the flow rectifying effect.
A sufficiently elongated straight passage could make stable the fluid flowing therethrough. Such an elongated passage, however, could not be often applied to apparatuses owing to a limitation of spaces. For example, with a fuel injection system for an automobile engine, a length of the passage in which a flow meter is located is extremely limited in the order of 20 cm because of various equipment arranged in a confined space.
Flow rectifiers have been used for this purpose, which include a number of rectifier elements or cells in the form of honeycomb, grate or the like.
The smaller the size of the cells, the greater is the rectifying effect of the rectifier but the larger is the loss of pressure. As the increasing rate of the loss of pressure is substantially proportional to a square on a reciprocal of the size of the cells, the loss of pressure rapidly increases as the size of the cells is smaller. Smaller cells than those sufficient to obtain the required accuracy of measurement would only increase the loss of pressure.
In addition, the rectifying effect of the flow rectifier is generally affected by a distance between it and a flow meter. The flow immediately downstream of the flow rectifier is not sufficiently rectified, whereas the flow remote from the rectifier is again distrubed in its longer passage although it has been once rectified.